Users don't see that stuff unless they want to. Advertisers don't care unless their ads appear next to it. And these platforms have pretty much solved this. E.g. Google search results contain the full cesspit of the internet, yet everyone advertises with Google.

The issue came when the news media decided that it was newsworthy that undesirables we're saying undesirable things on social media and blamed the platform as a whole. Now it's a pr issue for advertisers to be on the platform at all whether or not they are adjacent to undesirable content.